In a wiring board such as a multilayered high-density wiring board, through-holes have conventionally been formed with a mechanical process using a drill or the like. However, the mechanical processing is not readily applicable under the recent circumstances in which the wiring density is increased, the diameter of though-hole is made smaller, and the pitch between through-holes is also made smaller in the wiring board. For the purposes of solving these problems and further improving the processing efficiency, there is a tendency to employ laser processing with a laser beam. In the case that a pulsed laser is used to form a through-hole, however, the through-hole has a tapered shape as shown in FIG. 10 (a), (b), causing a problem of defective plating for example. Furthermore, scattered fragments, burrs and the like are generated as shown in FIG. 11 (a), (b), causing a problem of increase in number of processes for rework, for example. In order to solve these problems, it is desired to develop a high-precision technique for forming through-holes by means of a pulsed laser. As for FIG. 10 (a), (b) and FIG. 11 (a), (b), a detailed description will be given in connection with Examples.
In order to meet the requirements as described above, there have been proposed laser processing methods improved in accuracy. For example, there is a proposed method in which a laser beam is applied to each of the surfaces of a workpiece in a process of forming a through-hole, so as to make the through-hole straight-like (approach to a right circular cylinder, or reduction in taper angle) (Patent Document 1). With this method, the diameters on the front and back sides of through-hole can be made almost equal to each other. Thus, it becomes possible to reduce the taper angle of through-hole and then to form a straight-like through-hole.
There is another proposed processing method that utilizes coherent laser light reflected from a workpiece in a laser ablation process so as to make it possible to readily form a through-hole having a reduced difference between the diameters on its front and back sides (Patent Document 2). With this method, the reflected light can increase the energy density of light used for processing and change the shape of through-hole, whereby making it possible to arrange through-holes at a high density.    Patent Document 1: WO99/59761    Patent Document 2: Japanese Patent Laying-Open No. 2000-77824